1. Field of the Invention
The present invention relates to electrometallurgy, particularly to plasma metallurgy and more specifically it pertains to a technique of the plasma jet remelting of a surface layer of a flat metal work having parallel side edges as well as to an apparatus for carrying out the remelting process.
The term "flat work having parallel side edges" means herein a work having at least one flat surface bounded by two parallel lines. Following this definition, a prismatic work of a rectangular cross-section or a semicylindrical work each will be termed a flat work.
2. Description of the Prior Art
Metallurgical semi-products, specifically those produced by continuous casting of metals, are known to have in their surface layer various defects such as cracks, blisters, peeling, coarse non-metallic inclusions, scale and so on.
Therefore a work, prior to the next process step, i.e. plastic deformation, e.g. rolling, is subjected to machining with the aim of removing a defective surface layer.
Such machining usually includes milling, planing, and removing the surface layer by means of abrasives, which results in a considerable wastage of metal (up to 30%).
Reprocessing of such waste metal, e.g. chips, by compacting and consequent remelting involves extra time and cost. The metal wastage therewith is still rather high.
The metal wastage becomes particularly appreciable when costly materials are involved. Furthermore, rapid wear of cutting tools is evident in machining hard alloys, which results in a lower efficiency of labor and a higher cost of the process.
It is to be noted that machining, as a surface defect removing technique, is a prolonged procedure. A low efficiency of labor is the result.
It might be well to emphasize that use of such technique at iron and steel works requires an extensive floor space to install roughing and dressing machines.
As compared with the mechanical elimination of defects in the surface layer of a work, a method of refining this layer which consists in the plasma jet remelting of this layer proves to be more efficient. This technique provides for the complete elimination of defects in the surface layer of a work for practically all purposes with a minimum metal wastage. This treatment results in a higher density of the work metal, lower contents of gas entrapments and non-metallic inclusions.
Known in the art is a method of plasma jet remelting of a surface layer of a metal work which is specifically a flat work having parallel side edges as disclosed in British Pat. No. 1,305,671, Cl. C7D, published in 1973, French Pat. No. 2,096,550, Int.Cl. C22d 7/00, published in 1974, and FRG Pat. No. 2,121,439, Int.Cl. C21c 5/52, published in 1975. This method includes forming a metal pool on a flat work positioned in a mold by heating the work through the use of plasma torches supplied with a plasma forming gas and electric current, while relatively moving the work and the plasma torches along the parallel edges of the work.
In accordance with the method a prismatic work of a rectangular cross-section is positioned, in the starting position, vertically in a mold so that the mold encloses on the work around the whole profile thereof.
The plasma torches are arranged around the work in the plane of the profile thereof so that each plasma torch faces a respective face of the work.
The work is placed on a starting plate and, as the periphery of the work is remelted by means of the plasma torches, is vertically drawn through the mold. In the process of remelting the level of the metal pool formed for each face of the work, is perpendicular to the surface being treated.
Hereinabove described method of the plasma jet remelting of a surface layer of a flat metal work having parallel side edges may be practiced by the use of an apparatus disclosed in British Pat. No. 1,237,115, Cl. B3F, published in 1971, and French Pat. No. 1,579,039, B22d 11/00 published in 1969.
This apparatus comprises a sealed chamber with a mold adapted to receive a work and plasma torches arranged within the chamber and through supply lines connected with sources of a plasma forming gas, electric current, and cooling water.
The mold of said apparatus has a form of a rectangular frame enclosing on the work around the periphery thereof. The opposing members of the frame are two cooled beams lying in a horizontal plane and being parallel to one another.
Mounted between the beams of the mold is a starting plate adapted to withdraw the ingot as it is formed from the work.
The plasma torches are arranged within the sealed chamber partially, i.e. only nozzle portions thereof are within the chamber, and are fixed in relation to this chamber.
The practice of the above-described method through the use of the above apparatus presents some difficulties, which are as follows.
According to the prior art technique the remelted surface is formed by contact thereof with the cooled mold. In doing this, some coarse defects are apt to appear in the macrostructure of the remelted layer, e.g. lateral cracks and longitudinal porosity.
It will be noted that the metal at the surface of the work is maintained liquid simultaneously with an intensive cooling action of the mold surrounding the metal pool around the work. In this case, if a relatively shallow metal pool is to be maintained in an effort to remelt the surface layer corresponding to the thickness of a defective layer, the metal pool will partially solidify thus forming several pools subjected to the action of the plasma torches and divided by solidified metal.
In a further drawing of the work through the mold, non-remelted portions of metal containing the defective layer will remain on the work surface. Consequent deformation of such work will reveal the defects of its surface layer and the work will be rejected.
Therefore the surface layer is to be remelted to a depth of 5 to 10 thicknesses of the defective layer. This results in overexpenditure of electric power and the plasma forming gas, as well as in decreased efficiency and an increased cost of the plasma jet remelting process.
In remelting the surface layer of a work according to the prior art technique, liquid metal solidifies in such a manner that the axes of pulled crystals in the layer are directed to the ingot surface at a substantially right angle. Such macrostructure of the surface layer remains even in the case of varying the power of the plasma torches and the speed of withdrawing the work.
Such macrostructure is responsible for deteriorations of plastic properties of the metal. Further deformation of the work involves crack formation between the pulled crystals, since the direction of the deformation force and the orientation of the axes of these crystals coincide.
It is also well to emphasize that according to the prior art method the thickness of the remelted layer of the work is determined by the width of the metal pool on the surface of the work. The level of the pool is not a practical measure for determining the thickness of the remelted layer.
That is why an effort to intensify the process of metal refining by widening the metal pool results in a deeper melting of the work and thus in a higher consumption of electric power and the plasma forming gas.
On the other hand, an increase in productivity of the treatment through raising the speed of withdrawing the work from the mold results in a poor surface of the work thus formed. This is caused since an increase in the speed of withdrawing the work with the thickness of the layer remelted being unchanged (which is attainable through an increase in the plasma torch power) results in the metal pool elongation in the direction of the work movement. In this case the solidification front acquires the form of a wedge. The crystal growth is directed from the wedge faces to the axis of the wedge. The crystal growth from the opposite directions involves, in a solidified work, porosity formation along the wedge axis and impairs the quality of the work surface layer.
For the reasons hereinabove stated the prior art technique provides for the production of the work with a remelted layer of an adequate quality and a thickness at least sufficient to eliminate defects in the initial layer only at a relatively low productivity which does not meet the requirements of the present-day metallurgical production. Actually the rate of withdrawal of the work from the mold is under 20 mm/min.
It is to be also noted that an effort to control the thickness of the layer being remelted by controlling the pool level through the variably rated plasma torches yields no substantial results, since the metal pool level is not controlling the thickness of the layer.
And finally, reference should be made to the fact that the practice of the above method on the prior art apparatus reveals instability of the plasma jet remelting, i.e. the pool level fluctuates in the mold and the overfilling of the mold is likely to occur. Some amount of metal is therewith often transferred as drops from the surface being remelted to the pool through the plasma arc.
Drops of liquid metal that get into the plasma arc impair the arc stability and get splashed away which results in loss of metal and poor surface of the work.